U.S. patent application number 14/737876 was filed with the patent office on 2015-12-17 for soft magnetic member, reactor, powder for dust core, and method of producing dust core.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. The applicant listed for this patent is TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Takeshi HATTORI, Junghwan HWANG, Kohei ISHII, Masashi OHTSUBO, Daisuke OKAMOTO, Kiyotaka ONODERA, Shinjiro SAIGUSA, Masaaki TANI.
Application Number | 20150364235 14/737876 |
Document ID | / |
Family ID | 53404392 |
Filed Date | 2015-12-17 |
United States Patent
Application |
20150364235 |
Kind Code |
A1 |
OKAMOTO; Daisuke ; et
al. |
December 17, 2015 |
SOFT MAGNETIC MEMBER, REACTOR, POWDER FOR DUST CORE, AND METHOD OF
PRODUCING DUST CORE
Abstract
A soft magnetic member is formed such that, when a differential
relative permeability in an applied magnetic field of 100 A/m is
represented by a first differential relative permeability .mu.'L,
and when a differential relative permeability in an applied
magnetic field of 40 kA/m is represented by a second differential
relative permeability .mu.'H, a ratio of the first differential
relative permeability .mu.'L to the second differential relative
permeability .mu.'H satisfies a relationship of
.mu.'L/.mu.'H.ltoreq.10, and a magnetic flux density in an applied
magnetic field of 60 kA/m is 1.15 T or higher.
Inventors: |
OKAMOTO; Daisuke;
(Toyota-shi, JP) ; ONODERA; Kiyotaka; (Toyota-shi,
JP) ; SAIGUSA; Shinjiro; (Toyota-shi, JP) ;
ISHII; Kohei; (Nagoya-shi, JP) ; OHTSUBO;
Masashi; (Nagakute-shi, JP) ; HWANG; Junghwan;
(Nagakute-shi, JP) ; TANI; Masaaki; (Nagakute-shi,
JP) ; HATTORI; Takeshi; (Nagakute-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TOYOTA JIDOSHA KABUSHIKI KAISHA |
Toyota-shi |
|
JP |
|
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi
JP
|
Family ID: |
53404392 |
Appl. No.: |
14/737876 |
Filed: |
June 12, 2015 |
Current U.S.
Class: |
336/221 ;
335/296; 335/297; 419/38 |
Current CPC
Class: |
H01F 1/14791 20130101;
B22F 2998/10 20130101; C22C 33/0257 20130101; B22F 9/08 20130101;
B22F 1/02 20130101; B22F 9/04 20130101; B22F 2009/0828 20130101;
B22F 3/02 20130101; B22F 3/10 20130101; B22F 2998/10 20130101; H01F
27/255 20130101; H01F 41/0246 20130101; H01F 3/08 20130101; H01F
1/24 20130101; H01F 1/33 20130101; C22C 33/0264 20130101; B22F 1/02
20130101 |
International
Class: |
H01F 1/24 20060101
H01F001/24; B22F 3/12 20060101 B22F003/12; H01F 27/255 20060101
H01F027/255; H01F 1/147 20060101 H01F001/147; H01F 41/02 20060101
H01F041/02; H01F 3/08 20060101 H01F003/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 13, 2014 |
JP |
2014-122429 |
Claims
1. A soft magnetic member wherein the soft magnetic member is
formed such that, when a differential relative permeability in an
applied magnetic field of 100 A/m is represented by a first
differential relative permeability .mu.'L, and when a differential
relative permeability in an applied magnetic field of 40 kA/m is
represented by a second differential relative permeability .mu.'H,
a ratio of the first differential relative permeability .mu.'L to
the second differential relative permeability .mu.'H satisfies a
relationship of .mu.'L/.mu.'H.ltoreq.10, and a magnetic flux
density in an applied magnetic field of 60 kA/m is 1.15 T or
higher.
2. The soft magnetic member according to claim 1, wherein the soft
magnetic member is a dust core formed from a powder for a dust
core, in the powder for the dust core, surfaces of soft magnetic
particles are coated with an insulating film, and the insulating
film has a Vickers hardness, which is 2.0 times or higher than that
of the soft magnetic particles, and has a thickness of 150 nm to 2
.mu.m.
3. The soft magnetic member according to claim 2, wherein the soft
magnetic particles are formed of an iron-aluminum-silicon alloy,
and the insulating film contains aluminum oxide as a major
component.
4. A reactor comprising: a core formed of the dust core that is the
soft magnetic member according to claim 2; and a coil that is wound
around the core.
5. A powder for a dust core wherein in the powder for the dust
core, surfaces of soft magnetic particles are coated with an
insulating film, and the insulating film has a Vickers hardness,
which is 2.0 times or higher than that of the soft magnetic
particles, and has a thickness of 150 nm to 2 .mu.m.
6. The powder for the dust core according to claim 5 wherein the
soft magnetic particles are formed of an iron-aluminum-silicon
alloy, and the insulating film contains aluminum oxide as a major
component.
7. A method of producing a dust core, the method comprising:
forming a green compact from the powder for the dust core according
to claim 5, and; sintering the green compact.
Description
INCORPORATION BY REFERENCE
[0001] The disclosure of Japanese Patent Application No.
2014-122429 filed on Jun. 13, 2014 including the specification,
drawings and abstract is incorporated herein by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a soft magnetic member
having superior magnetic characteristics, a reactor using the soft
magnetic member, a powder for a dust core, and a method of
producing a dust core.
[0004] 2. Description of Related Art
[0005] In a hybrid vehicle, an electric vehicle, a solar power
generation device, or the like, a reactor is used, and this reactor
adopts a structure in which a coil is wound around a ring-shaped
core which is a soft magnetic member. During use of the reactor, a
wide range of currents flow through the coil. Therefore, at least
40 kA/m of magnetic field is applied to the core. In such an
environment, it is necessary to stably secure the inductance of the
reactor.
[0006] In consideration of the above-described points, for example,
a reactor 9 is disclosed in which, as shown in FIG. 9A, a
ring-shaped core 91 is divided into core portions 92A, 92B, a gap
93 is provided between the divided core portions 92A, 92B, and
coils 95A, 95B are wound around the core 91 including this gap 93
(for example, refer to Japanese Patent Application Publication No.
2009-296015 (JP 2009-296015 A)).
[0007] According to the reactor 9, the gap 93 is provided between
the divided core portions 92A, 92B; as a result, even when a wide
range of currents flow through the coil 95 of the reactor 9, the
inductance can be stably secured in this wide range of
currents.
[0008] However, a soft magnetic member is used in a choke coil, an
inductor, or the like. As such a soft magnetic member, a dust core
is disclosed in which, when an initial magnetic permeability is
represented by .mu..sub.0 and a magnetic permeability in an applied
magnetic field of 24 kA/m is represented by a relationship of
.mu./.mu..sub.0.gtoreq.0.5 is satisfied between .mu..sub.0 and .mu.
(for example, refer to Japanese Patent Application Publication No.
2002-141213 (JP 2002-141213 A)). According to this dust core, even
if a high magnetic field is applied to the dust core, a decrease in
the magnetic permeability of the dust core can be suppressed.
[0009] However, for example, in the technique disclosed in JP
2009-296015 A, the gap is formed between the divided core portions.
Therefore, as shown in FIG. 9B, a magnetic flux T is leaked in the
gap 93 formed between the divided core portions 92A, 92B. In
particular, in a reactor of a hybrid vehicle or the like through
which a high current flows, a high magnetic field of about 40 kA/m
is applied to a core. Therefore, in order to maintain the
inductance of the reactor (that is, the core) at the applied
magnetic field, it is necessary to further increase the
above-described gap. As a result, the leakage of the magnetic flux
T from the gap is increased, and this leaked magnetic flux
intersects with the coil, which causes eddy-current loss in the
core.
[0010] The problem which is described above using the reactor is an
example. In equipment or an apparatus in which a magnetic field in
a range from a low magnetic field to a high magnetic field (40
kA/m) is applied to a soft magnetic member, it is difficult to
maintain the inductance, and typically a structural measure is
taken.
[0011] Even if a soft magnetic member having the characteristics
disclosed in JP 2002-141213 A is used, as clearly seen from an
experiment of the present inventors described below, the
application of a high magnetic field of about 40 kA/m is not
considered. Therefore, even if such a material is used, a
significant decrease in inductance is assumed in a high magnetic
field (about 40 kA/m).
SUMMARY OF THE INVENTION
[0012] The invention provides a soft magnetic member, a reactor, a
powder for a dust core, and a method of producing a dust core, in
which a decrease in inductance can be suppressed even if an applied
magnetic field is high (about 40 kA/m).
[0013] As a result of a thorough study, the present inventors
thought that, in order to suppress a decrease in inductance in a
high magnetic field, it is important to secure a predetermined
amount of magnetic flux density and to adjust a differential
relative permeability to be high even in a high magnetic field.
Therefore, the present inventors have focused on a ratio of a
differential relative permeability in a specific low magnetic field
to a differential relative permeability in a specific high magnetic
field.
[0014] According to a first aspect of the invention, there is
provided a soft magnetic member, in which when a differential
relative permeability in an applied magnetic field of 100 A/m is
represented by a first differential relative permeability .mu.'L,
and when a differential relative permeability in an applied
magnetic field of 40 kA/m is represented by a second differential
relative permeability .mu.'H, a ratio of the first differential
relative permeability .mu.'L to the second differential relative
permeability .mu.'H satisfies a relationship of
.mu.'L/.mu.'H.ltoreq.10, and a magnetic flux density in an applied
magnetic field of 60 kA/m is 1.15 T or higher.
[0015] In the soft magnetic member according to the aspect of the
invention, the ratio of the first differential relative
permeability .mu.'L to the second differential relative
permeability .mu.'H satisfies a relationship of
.mu.'L/.mu.'H.ltoreq.10. As a result, the gradient of the B-H curve
of the soft magnetic member can be secured to be large even in a
high magnetic field, and the inductance of the soft magnetic member
in a magnetic field of 40 kA/m can be maintained.
[0016] Here, when .mu.'L/.mu.'H>10, a difference in differential
relative permeability between a low magnetic field and a high
magnetic field is increased. As a result, when a magnetic field is
applied to a high magnetic field region, a decrease in inductance
is increased. For example, when a core is divided into portions in
a reactor, it is necessary that a gap between the divided portions
be increased in order to maintain the inductance of the reactor. As
a result, a leakage of a magnetic flux from the gap is increased,
and this leaked magnetic flux intersects with the coil, which
causes eddy-current loss in the core. It is preferable that
.mu.'L/.mu.'H is low, and the lower limit thereof is 1. When
.mu.'L/.mu.'H<1, it is difficult to produce a soft magnetic
member.
[0017] In addition, a magnetic flux density of 1.15 T or higher is
secured in an applied magnetic field of 60 kA/m, and thus the
inductance value can be maintained in a range from a low magnetic
field to a high magnetic field. That is, when the magnetic flux
density in an applied magnetic field of 60 kA/m is lower than 1.15
T, a decrease in inductance in a range from a low magnetic field to
a high magnetic field is a concern. Therefore, this soft magnetic
member is not sufficient for use in equipment such as a reactor.
The upper limit of the magnetic flux density in an applied magnetic
field of 60 kA/m is preferably 2.1 T. Since the saturated magnetic
flux density of pure iron is about 2.2 T, it is difficult to
produce a soft magnetic member having a magnetic flux density of
more than 2.2 T.
[0018] Here, "differential relative permeability" described herein
is obtained by dividing a gradient of a tangent to a curve (B-H
curve) between a magnetic field H and a magnetic flux density B by
a space permeability, the curve being obtained by continuously
applying a magnetic field. For example, a differential relative
permeability in a magnetic field of 40 kA/m (second differential
relative permeability .mu.'H) is obtained by dividing a gradient of
a tangent to a B-H curve in a magnetic field of 40 kA/m by a space
permeability.
[0019] In the soft magnetic member according to the aspect of the
invention, the soft magnetic member may be a dust core formed from
a powder for the dust core; in the powder for the dust core,
surfaces of soft magnetic particles may be coated with an
insulating film; and the insulating film may have a Vickers
hardness, which is 2.0 times or higher than that of the soft
magnetic particles, and may have a thickness of 150 nm to 2
.mu.m.
[0020] As clearly seen from an experiment of the present inventors
described below, when a compact as a dust core is formed, a
material constituting the insulating film is not likely to be
unevenly distributed in a boundary (triple point) between three
particles of a powder for a dust core by adjusting the Vickers
hardness and the thickness of the insulating film to be in the
above-described ranges. As a result, after the formation of the
compact, the distance between soft magnetic particles is secured,
and a non-magnetic material as a material of the insulating film is
maintained between the soft magnetic particles.
[0021] In a dust core obtained by sintering the compact obtained as
described above, the magnetic flux density during the application
of a low magnetic field to the soft magnetic member can be
decreased without decreasing the magnetic flux density in an
applied magnetic field of 60 kA/m. That is, even when a magnetic
field in a range from a low magnetic field (100 A/m) to a high
magnetic field (40 kA/m) is applied to the dust core, a decrease in
differential relative permeability in a high magnetic field can be
suppressed. As a result, the inductance of the dust core in the
above-described applied magnetic field range can be maintained.
[0022] Here, when the Vickers hardness of the insulating film is
lower than two times that of the soft magnetic particles, a
material constituting the insulating film is likely to be unevenly
distributed in a boundary (triple point) between three particles of
a powder for a dust core during the formation of the powder. When
the Vickers hardness of the insulating film is higher than 20 times
that of the soft magnetic particles, the insulating film is too
hard to compression-form a powder for a dust core.
[0023] When the thickness of the insulating film is less than 150
nm, the distance between the soft magnetic particles cannot be
sufficiently secured, which may increase .mu.'L/.mu.'H. On the
other hand, when the thickness of the insulating film exceeds 2
.mu.m, an occupancy of a non-magnetic component (insulating film)
increases, and thus it is difficult to satisfy a relationship in
which the magnetic flux density in an applied magnetic field of 60
kA/m is 1.15 T or higher.
[0024] Further, in the above-described aspect, the soft magnetic
particles may be formed of an iron-aluminum-silicon alloy, and the
insulating film may contain aluminum oxide as a major component. By
selecting such a material, the above-described relationship of
.mu.'L/.mu.'H.ltoreq.10 is satisfied, and the condition where the
magnetic flux density in an applied magnetic field of 60 kA/m is
1.15 T or higher is likely to be satisfied.
[0025] In particular, when aluminum of the soft magnetic particles
formed of an iron-aluminum-silicon alloy is preferentially oxidized
by oxidizing gas having a predetermined gas ratio, the
above-described hardness relationship and the above-described
thickness range can be easily satisfied.
[0026] According to a second aspect of the invention, there is
provided a reactor. The reactor includes: a core formed of the
above-described dust core; and a coil that is wound around the
core. In such a reactor, even when a current in a range from a low
current to a high current flows through the coil, the inductance is
maintained. Therefore, the core is not necessarily divided, or even
if the core is divided into portions, a gap between the divided
portions can be reduced. As a result, eddy-current loss of the coil
due to a leaked magnetic flux can be removed or decreased.
[0027] Further, according to a third aspect of the invention, there
is provided a powder for a dust core which is suitable for the
above-described dust core. In the powder for the dust core
according to the third aspect of the invention, surfaces of soft
magnetic particles may be coated with an insulating film, and the
insulating film may have a Vickers hardness, which is 2.0 times or
higher than that of the soft magnetic particles, and may have a
thickness of 150 nm to 2 .mu.m.
[0028] By using the powder for a dust core, the relationship of
.mu.'L/.mu.'H.ltoreq.10 can be satisfied, and a powder for a dust
core having an magnetic flux density of 1.15 T or higher in an
applied magnetic field of 60 kA/m can be easily produced.
[0029] In the above-described aspect, the soft magnetic particles
may be formed of an iron-aluminum-silicon alloy, and the insulating
film may contain aluminum oxide as a major component. In
particular, when aluminum of the soft magnetic particles formed of
an iron-aluminum-silicon alloy is preferentially oxidized by
oxidizing gas having a predetermined gas ratio, the above-described
hardness relationship and the above-described thickness range can
be easily satisfied.
[0030] According to a fourth aspect of the invention, there is
provided a method of producing a dust core including: forming a
green compact from the powder for the dust core according to the
above-described aspect of the invention, and; sintering the green
compact. As a result, a dust core having the above-described
characteristics can be obtained.
[0031] According to the above-described aspects of the invention,
even when the applied magnetic field is high (about 40 kA/m), a
decrease in inductance can be suppressed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] Features, advantages, and technical and industrial
significance of exemplary embodiments of the invention will be
described below with reference to the accompanying drawings, in
which like numerals denote like elements, and wherein:
[0033] FIGS. 1A to 1C are schematic diagrams showing a method of
producing a soft magnetic member (dust core) according to an
embodiment of the invention, in which FIG. 1A is a diagram showing
soft magnetic particles, FIG. 1B is a diagram showing particles
constituting a powder for a dust core, and FIG. 1C is a diagram
showing a particle state in a compact;
[0034] FIGS. 2A to 2D are schematic diagrams showing a method of
producing a soft magnetic member (dust core) of the related art, in
which FIG. 2A is a diagram showing soft magnetic particles, FIG. 2B
is a diagram showing particles constituting a powder for a dust
core, FIG. 2C is a diagram showing a particle state in a compact,
and FIG. 2D is an enlarged image showing a dust core produced using
the method of the related art;
[0035] FIG. 3A is a diagram showing a relationship between an
applied magnetic field and a magnetic flux density in each of
Conventional Product 1 and Conventional Product 2 in which the
amount of a resin is more than that of Conventional Product 1;
[0036] FIG. 3B is a diagram showing a relationship between an
applied magnetic field and a magnetic flux density in each of
Conventional Product 1 and a product of the invention;
[0037] FIG. 4 is a B-H curve diagram showing ring test pieces of
Example 1 and Comparative Example 1;
[0038] FIG. 5 is a diagram showing a relationship between an
inductance and a DC superimposed current in each of reactors of
Example 1 and Comparative Example 1;
[0039] FIG. 6 is a B-H curve diagram showing ring test pieces of
Examples 1 to 7 and Comparative Examples 2 to 6;
[0040] FIG. 7 is a diagram showing a relationship between
.mu.'L/.mu.'H and a magnetic flux density B in an applied magnetic
field of 60 kA/m in each of ring test pieces of Examples 1 to 7 and
Comparative Examples 1 to 6;
[0041] FIG. 8A is a diagram showing a relationship between
.mu.'L/.mu.'H and a ratio of the hardness of an insulating film of
a powder for a dust core used in each of the ring test pieces of
Examples 1 to 7 and Comparative Examples 1 to 3;
[0042] FIG. 8B is a diagram showing a relationship between
.mu.'L/.mu.'H and the thickness of the insulating film of the
powder for a dust core used in each of the ring test pieces of
Examples 1 to 7 and Comparative Examples 1 to 3; and
[0043] FIG. 9A is a schematic diagram showing a reactor of the
related art;
[0044] FIG. 9B is an enlarged view showing major components of the
reactor in FIG. 9A.
DETAILED DESCRIPTION OF EMBODIMENTS
[0045] Hereinafter, an embodiment of a powder for a dust core
according to the invention and a soft magnetic member formed from
the powder will be described with reference to the drawings.
[0046] FIGS. 1A to 1C are schematic diagrams showing a method of
producing a soft magnetic member (dust core) according to an
embodiment of the invention, in which FIG. 1A is a diagram showing
soft magnetic particles, FIG. 1B is a diagram showing particles
constituting a powder for a dust core, and FIG. 1C is a diagram
showing a particle state in a compact;
[0047] As shown in FIG. 1B, the powder 10 for a dust core according
to the embodiment is an aggregate of particles 13 for a dust core.
The particles 13 for a dust core include: soft magnetic particles
11 formed of a soft magnetic material; and an insulating film 12
formed of a non-magnetic material, in which surfaces of the soft
magnetic particles 11 are coated with the insulating film 12, and
the insulating film has a hardness, which is 2 times or higher than
that of the soft magnetic particles 11, and has a thickness of 150
nm to 2 .mu.m.
[0048] The average particle size of particles (on which the
insulating film is formed) constituting the powder 10 for a dust
core is preferably 5 .mu.m to 500 .mu.m and more preferably 20
.mu.m to 450 .mu.m. By using the soft magnetic powder having an
average particle size in the above-described range, a dust core
having superior insulating properties can be obtained. When the
average particle size is less than 20 .mu.m, a ratio of an
insulating material constituting the insulating film is increased,
which decreases the saturated magnetic flux density. On the other
hand, when the average particle size is more than 450 .mu.m, a
ratio of an insulating material constituting the insulating film is
decreased, and it is difficult to obtain desired magnetic
characteristics and desired insulating properties (specific
resistance). When the average particle size is more than 500 .mu.m,
it is difficult to obtain insulating properties, and the eddy
current of the particles (powder) is increased, and the loss is
increased.
[0049] A method of producing the powder 10 for a dust core will be
described below. First, as shown in FIG. 1A, as the soft magnetic
material constituting the soft magnetic particles (base particles)
11, for example, iron, cobalt, or nickel is prepared. More
preferably, an iron-based material may be used, and examples
thereof include iron (pure iron), an iron-silicon alloy, an
iron-nitrogen alloy, an iron-nickel alloy, an iron-carbon alloy, an
iron-boron alloy, an iron-cobalt alloy, an iron-phosphorus alloy,
an iron-nickel-cobalt alloy, and an iron-aluminum-silicon
alloy.
[0050] Examples of the soft magnetic powder formed of the soft
magnetic particles 11 include water-atomized powder, gas-atomized
powder, and pulverized powder. From the viewpoint of suppressing a
destruction of an insulating layer during press forming, it is more
preferable to select a powder having a small amount of
convexo-concave portions on particle surfaces.
[0051] When the above-described metals are selected as the soft
magnetic material constituting the soft magnetic particles 11, for
example, iron oxide (Fe.sub.3O.sub.4, Fe.sub.2O.sub.3), iron
nitride, silicon oxide (SiO.sub.2), or silicon nitride
(Si.sub.3O.sub.4) can be used as the material of the insulating
film 12 under the condition that the above-described thickness
range and the above-described hardness relationship of the film are
satisfied. As another conditions, it is necessary that the formed
dust core satisfy a relationship of .mu.'L/.mu.'H.ltoreq.10
described below and satisfy a magnetic flux density of 1.15 T or
higher in an applied magnetic field of 60 kA/m.
[0052] In addition, the insulating film 12 can be formed on the
soft magnetic particles 11 by oxidizing the surfaces of the soft
magnetic particles 11 shown in FIG. 1A. As another method, the
above-described material constituting the insulating film may be
attached on the surfaces of the soft magnetic particles 11 shown in
FIG. 1A using PVD, CVD, or the like.
[0053] In the embodiment, an iron-aluminum-silicon alloy is used as
the soft magnetic material constituting the soft magnetic particles
11. The soft magnetic particles 11 formed of the metal alloy are
oxidized by being heated using a mixed oxidizing gas containing
nitrogen gas and oxygen gas at a predetermined ratio, the gases
being supplied from industrial gas cylinders. At this time,
aluminum is dispersed and compressed on the surfaces of the soft
magnetic particles 11, and aluminum is preferentially oxidized.
[0054] As a result, a film containing aluminum oxide having a high
purity as a major component (containing aluminum oxide and
unavoidable impurities) can be formed. Aluminum oxide has higher
hardness and insulating properties than those of other materials,
is superior in heat resistance, and is highly stable to a chemical
solution such as a coolant. As a result, the insulating film 12
formed of aluminum oxide, which has a hardness two times or higher
than that of the soft magnetic particles 11 and has a thickness of
150 nm to 2 .mu.m, can be easily obtained.
[0055] Here, in the iron-aluminum-silicon alloy, it is preferable
that the Si content is 1 mass % to 7 mass %, the Al content is 1
mass % to 6 mass %, the total content of Si and Al is 1 mass % to
12 mass %, and the balance includes iron and unavoidable
impurities.
[0056] Here, when the contents of Si and Al are lower than the
above-described ranges, it is difficult to produce aluminum oxide,
other oxides are produced, and thus magnetic loss increases. In
addition, when the Si content exceeds the above-described range,
the plastic deformation resistance of the powder for a dust core
increases, and the formability into a dust core deteriorates.
Therefore, the saturated magnetic flux density decreases. In
addition, when the total content of Si and Al exceeds the
above-described range, or when the Al content exceeds the
above-described range, a ratio of iron in the soft magnetic
particles decreases, and the saturated magnetic flux density
decreases.
[0057] As shown in FIG. 1C, the powder for a dust core is
compression-formed into a green compact, and this green compact is
annealed by a heat treatment. As a result, a dust core 1 can be
obtained. At this time, the insulating film 12, which has a
hardness two times or higher than that of the soft magnetic
particles 11 and has a thickness of 150 nm to 2 .mu.m, is provided.
Therefore, the material (non-magnetic material) constituting the
insulating film 12 is not likely to be distributed in a boundary 14
(triple point) between three particles 13, 13, 13 (base material)
for a dust core. As a result, after the formation of the compact,
the distance between the soft magnetic particles 11, 11 is secured,
and the non-magnetic material as the material of the insulating
film 12 is maintained between the soft magnetic particles.
[0058] In the related art, as shown in FIG. 2B, a powder 80 for a
dust core formed of particles 83 for a dust core is used, in which
surfaces of soft magnetic particles 81 are coated with a soft
insulating film 82 formed of a silicone resin or the like. When a
magnetic field in a range from a low magnetic field to a high
magnetic field is applied to a dust core 8 of FIG. 2C produced
using the powder 80 for a dust core, in a high magnetic field
(exceeding 40 kA/m), the magnetic flux density approaches the
saturated magnetic flux density, and the differential relative
permeability decreases.
[0059] The inductance L of the dust core (reactor) is represented
by L=nS.mu.' (wherein n: the winding number of the coil, S: the
cross-sectional area of a portion of the dust core around which the
core is wound, .mu.': the differential relative permeability). In
order to maintain characteristics of the inductance L in a high
magnetic field, it is important to suppress a decrease in
differential relative permeability in a high magnetic field.
[0060] Here, the magnetic field H applied to the dust core is
represented by H=nI/L (wherein n: the winding number of the coil,
I: the current flowing through the coil, L: the magnetic path
length of the dust core), in which the current I flowing through
the coil is proportional to the applied magnetic field H.
Accordingly, in the dust core 8 (Conventional Product 1) shown in
FIG. 3A, in order to suppress a decrease in differential relative
permeability in a high magnetic field, a decrease in differential
relative permeability in a low magnetic field is effective.
[0061] Therefore, in Conventional Product 1, when the thickness of
the insulating film 82 shown in FIG. 2B is increased (when a ratio
of a resin is increased), the differential relative permeability in
a low magnetic field can be decreased by increasing the content of
the resin as a non-magnetic component. However, in Conventional
Product 2 of FIG. 3A, the saturated magnetic flux density in a high
magnetic field is decreased.
[0062] One of the reasons is presumed to be as follows: as shown in
FIG. 2C, a material (non-magnetic material) constituting the
insulating film 82 is unevenly distributed in the boundary (triple
point) 84 between three particles 83, 83, 83 for a dust core when a
compact is formed using the powder 80 for a dust core. As shown in
FIG. 2D, the uneven distribution of the resin in the triple point
was verified from an experiment of the present inventors.
[0063] From this point of view, it can be considered that, by
providing a gap in Conventional Product 1 (core) as shown in FIG.
9A, the magnetic flux density in a low magnetic field can be
decreased, and a decrease in differential relative permeability in
a high magnetic field can be decreased as shown in Conventional
Product 1 (gap provided) of FIG. 3B. However, when such a gap is
provided, a leakage of a magnetic flux T from the gap is increased
as shown in FIG. 9B, this leaked magnetic flux intersects with the
coil, which causes eddy-current loss in the core.
[0064] In the embodiment, the hardness and the thickness of the
insulating film 12 shown in FIG. 1B are adjusted to be in the
above-described ranges. As a result, when a compact as the dust
core 1 is formed, the material (non-magnetic material) constituting
the insulating film 12 is not likely to be unevenly distributed in
the boundary (triple point) 14 between three particles of the
powder 10 for a dust core. As a result, after the formation of the
compact, the distance between the soft magnetic particles 11, 11 is
secured, and the non-magnetic material as the material of the
insulating film 12 is maintained between the soft magnetic
particles.
[0065] In the dust core 1 obtained by sintering the compact
obtained as described above, when a differential relative
permeability in an applied magnetic field of 100 A/m is represented
by a first differential relative permeability .mu.'L, and when a
differential relative permeability in an applied magnetic field of
40 kA/m is represented by a second differential relative
permeability .mu.'H, a ratio of the first differential relative
permeability .mu.'L to the second differential relative
permeability .mu.'H satisfies a relationship of
.mu.'L/.mu.'H.ltoreq.10, and a magnetic flux density in an applied
magnetic field of 60 kA/m is 1.15 T or higher.
[0066] As a result, as shown in a product of the invention of FIG.
3B, even when a magnetic field in a range from a low magnetic field
(100 A/m) to a high magnetic field (40 kA/m) is applied to the dust
core, a decrease in differential relative permeability in a high
magnetic field can be suppressed. As a result, the inductance of
the dust core (reactor) in the above-described applied magnetic
field range can be maintained.
[0067] In the embodiment, as shown in FIG. 9A, unlike the
techniques of the related art, it is not necessary to provide a
large gap between divided core portions. Therefore, a leakage of
the magnetic flux in a reactor can be suppressed.
[0068] Hereinafter, the invention will be described using
Examples.
Example 1
Preparation of Powder for Dust Core
[0069] As a soft magnetic powder constituting soft magnetic
particles, a water-atomized power (maximum particle size: 75 .mu.m;
measured using a measured sieve defined according to JIS-Z8801)
formed of an iron-silicon-aluminum alloy (Fe-5Si-4Al) containing 5
mass % of Si and 4 mass % of Al in addition to Fe was prepared.
[0070] Next, the water-atomized powder was heated at 900.degree. C.
for 300 minutes in an atmosphere of a mixed oxidizing gas
containing 20 vol % of oxygen gas and 80 vol % of nitrogen gas, the
gases being supplied from industrial gas cylinders. As a result,
surfaces of the soft magnetic particles were coated with a film
formed of aluminum oxide (Al.sub.2O.sub.3) having a thickness of
460 nm as an insulating film. The formation of aluminum oxide was
measured using XRD analysis, and the thickness was measured using
Auger spectroscopy analysis (AES).
<Preparation of Ring Test Piece (Dust Core)>
[0071] The powder for a dust core is put into a die, and a
ring-shaped green compact having an outer diameter of 39 mm, an
inner diameter of 30 mm, and a thickness of 5 mm was prepared using
a die lubrication warm forming method under conditions of a forming
temperature of 130.degree. C. and a forming pressure of 16
t/cm.sup.2. The formed green compact was heat-treated (sintered) in
a nitrogen atmosphere at 750.degree. C. for 30 minutes. As a
result, a ring test piece (dust core) was prepared.
Comparative Example 1
[0072] A ring test piece (dust core) was prepared using the same
method as that of Example 1. Comparative Example 1 was different
from Example 1, in that an iron-silicon alloy (Fe-3Si) powder
containing 3 mass % of Si in addition to Fe was used as a soft
magnetic powder constituting the soft magnetic particles, 0.5 mass
% of silicone resin was added to the powder, and soft magnetic
particles were coated with this film at a film-forming temperature
of 130.degree. C. for a film-forming time of 130 minutes to prepare
a powder for a dust core.
<Evaluation of Ring Test Piece>
[0073] Using an Autograph, coils were wound around each of the ring
test pieces prepared in Example 1 and Comparative Example 1 under
conditions of a winding number of 450 turns on an excitation side
and 90 turns on a detection side. Next, by causing a current to
flow through the coil, a magnetic field was applied so as to
linearly increase from 0 kA/m to 60 kA/m. At this time, the
magnetic flux density was measured using a DC magnetic flux meter.
The results are shown in FIG. 4. FIG. 4 is a B-H curve diagram of
the ring test pieces of Example 1 and Comparative Example 1.
[0074] From the obtained graph (B-H curve diagram) showing the
applied magnetic field and the magnetic flux density, the first
differential relative permeability .mu.'L in an applied magnetic
field of 100 A/m, the second differential relative permeability
.mu.'H in an applied magnetic field of 40 kA/m, and .mu.'L/.mu.'H
were calculated. The results are shown in Table 1. In addition,
regarding each of the ring test pieces of Example 1 and Comparative
Example 1, the magnetic flux density in an applied magnetic field
of 60 kA/m was measured. The results are shown in Table 1.
[0075] Specifically, the first differential relative permeability
.mu.'L is a value calculated by calculating a gradient
(.DELTA.B/.DELTA.H) of a line connecting two points around an
applied magnetic field of 100 A/m in the B-H curve of FIG. 4 and
dividing this gradient by a space permeability. Likewise, the
second differential relative permeability .mu.'H is a value
calculated by calculating a gradient (.DELTA.B/.DELTA.H) of a line
connecting two points around an applied magnetic field of 40 A/m in
the B-H curve of FIG. 4 and dividing this gradient by a space
permeability. .mu.'L/.mu.'H is a value of the first differential
relative permeability .mu.'L/the second differential relative
permeability .mu.'H.
TABLE-US-00001 TABLE 1 Comparative Example 1 Example 1 First
Differential Relative Permeability .mu.'L 45 151 Second
Differential Relative Permeability .mu.'H 11.1 6.5 .mu.'L/.mu.'H 4
23 Magnetic Flux Density (T) in Magnetic Field of 1.33 1.95 60
kA/m
[Result 1]
[0076] As shown in Table 1, in the ring test piece (dust core) of
Example 1, the ratio .mu.'L/.mu.'H of the first differential
relative permeability .mu.'L to the second differential relative
permeability .mu.'H was about 1/6 of that of Comparative Example
and was 10 or lower (specifically, 4). That is, it can be said
that, in the dust core of Example 1, a decrease in differential
relative permeability in a high magnetic field was suppressed as
compared to the dust core of Comparative Example 1.
[0077] The reason is presumed to be as follows. In the dust core of
Example 1, the powder for a dust core was used in which the soft
magnetic particles were coated with the insulating film formed of
aluminum oxide Al.sub.2O.sub.3. Therefore, during compression
forming, the insulating film is less likely to flow as compared to
that of Comparative Example 1 in which a silicone resin was used.
As a result, in the dust core of Example 1, as compared to that of
Comparative Example 1, the insulating film was secured between the
soft magnetic particles. Therefore, it is considered that, even
when the applied magnetic field was high, a decrease in
differential relative permeability was suppressed.
[0078] In addition, in the dust core of Example 1, the magnetic
flux density in an applied magnetic field of 60 kA/m was
sufficiently high at 1.15 T which was equivalent to that of
Comparative Example 1, and the first differential relative
permeability .mu.'L was suppressed to be low. As a result, it is
considered that the second differential relative permeability 11'H
was able to be maintained to be high, and the ratio .mu.'L/.mu.'H
of the first differential relative permeability .mu.'L to the
second differential relative permeability .mu.'H was able to
satisfy .mu.'L/.mu.'H.ltoreq.10.
<Measurement of Inductance>
[0079] Further, a core of a reactor was prepared from each of the
dust cores corresponding to Example 1 and Comparative Example 1.
Using this core, a reactor shown in FIG. 9A was prepared. When a DC
superimposed current was applied to the coil, the inductance of the
reactor was measured. The results are shown in FIG. 5. At this
time, the gap width of the core (dust core), the measured
inductance, the magnetic loss of the reactor, and the eddy-current
loss of the coil were measured. The results are shown in Table 2.
The current values in parentheses shown in Table 2 are current
values flowing through the coil during the measurement.
TABLE-US-00002 TABLE 2 Comparative Example 1 Example 1 Inductance L
(at 10 A) 174 .mu.H 165 .mu.H Inductance L (at 100 A) 128 .mu.H 138
.mu.H Inductance L (at 200 A) 90 .mu.H 73 .mu.H Gap Length 1.8 mm
2.4 mm Magnetic Loss (at 50 A) 102 W 128 W Eddy-Current Loss of
Coil 24 W 40 W
[Result 2]
[0080] As shown in FIG. 5 and Table 2, in the reactor of Example 1,
although the gap length was shorter than that of Comparative
Example 1 by 0.6 mm (decrease by 25%), the inductance value was
higher than that of Comparative Example 1 even in a high current
region of 150 A or higher (that is, in a high magnetic field). That
is, as shown in Table 1, it can be said that, in the dust core of
Example 1, a decrease in differential relative permeability in a
high magnetic field was suppressed as compared to the dust core of
Comparative Example 1.
[0081] In addition, as shown in Table 2, in the reactor of Example
1, the gap length of the core was shorter than that of Comparative
Example 1. As a result, it is considered that the leakage of the
magnetic flux between the core portions shown in FIG. 9B was
decreased, and the magnetic loss and the eddy-current loss of the
coil were decreased.
Examples 2 to 7
[0082] A ring test piece (dust core) was prepared using the same
method as that of Example 1. Examples 3 to 5 were different from
Example 1, in that, as shown in Table 3, a water-atomized power
formed of an iron-silicon-aluminum alloy (Fe-2Si-4Al) containing 2
mass % of Si and 4 mass % of Al in addition to Fe was used as a
soft magnetic powder constituting soft magnetic particles (base
particles).
[0083] In addition, Example 4 was further different from Example 1,
in that the forming surface pressure was changed to 8 t/cm.sup.2.
Example 5 was further different from Example 1, in that the forming
surface pressure was changed to 12 t/cm.sup.2. Example 7 was
further different from Example 1, in that the heating time in an
oxidizing atmosphere was changed to 120 minutes. In Examples 2 and
6, the production conditions were the same as those of Example 1.
Table 3 also shows the production conditions of Example 1 in order
to clearly see the differences in production conditions between the
ring test piece of Example 1 and the ring test pieces of Examples 2
to 7.
TABLE-US-00003 TABLE 3 Maximum Oxidizing Particle Conditions Size
of Tem- Forming Den- Base pera- Surface sity Base Material ture
Time Pressure (g/ Particles (.mu.m) (.degree. C.) (min)
(t/cm.sup.2) cm.sup.3) Example 1 Fe--5Si--4Al 75 900 300 16 6.20
Example 2 Fe--5Si--4Al 75 900 300 16 6.21 Example 3 Fe--2Si--4Al 75
900 300 16 6.79 Example 4 Fe--2Si--4Al 75 900 300 8 6.21 Example 5
Fe--2Si--4Al 75 900 300 12 6.57 Example 6 Fe--5Si--4Al 75 900 300
16 6.19 Example 7 Fe--5Si--4Al 75 900 120 16 6.20
Comparative Examples 2, 3
[0084] A ring test piece (dust core) was prepared using the same
method as that of Example 1. Comparative Examples 2 and 3 were
different from Example 1, in that as shown in Table 4, iron-silicon
alloy (Fe-3Si) powders having maximum particle sizes of 45 .mu.m
and 180 .mu.m, which contained 3 mass % of Si in addition to Fe,
were used as a soft magnetic powder constituting the base
particles, 0.5 mass % of silicon resin was added to the powders,
and soft magnetic particles were coated with this film at a
film-forming temperature of 170.degree. C. for a film-forming time
of 170 minutes to prepare powders for a dust core. Table 4 also
shows the production conditions of Comparative Example 1 in order
to clearly see the differences in production conditions between the
dust core of Comparative Example 1 and the dust cores of
Comparative Examples 2 and 3.
TABLE-US-00004 TABLE 4 Maximum Coating Conditions Forming Particle
Content Film-Forming Film-Forming Surface Base Size of Base of
Resin Temperature Time Pressure Density Particles Material (.mu.m)
(mass %) (.degree. C.) (min) (t/cm.sup.2) (g/cm.sup.3) Comparative
Fe--3Si 180 0.5 130 130 16 7.25 Example 1 Comparative Fe--3Si 45
0.5 170 170 16 7.25 Example 2 Comparative Fe--3Si 180 0.5 170 170
16 7.30 Example 3
Comparative Examples 4, 5
[0085] In Comparative Example 4, as shown in Table 5, as a soft
magnetic powder constituting the soft magnetic particles, an
iron-silicon alloy (Fe-6.5Si) powder containing 6.5 mass % of Si in
addition to Fe was prepared, the soft magnetic powder was kneaded
with a polyphenylene sulfide (PPS) resin such that the content of
the PPS resin was 65 vol %, and the kneaded material was injected
into the same size and the same shape as those of Example 1. As a
result, a ring test piece was prepared.
[0086] In Comparative Example 5, a ring test piece was prepared by
injection molding using the same method as that of Comparative
Example 4. Comparative Example 5 was different from Comparative
Example 4, in that, as shown in Table 5, the soft magnetic powder
was kneaded with a polyphenylene sulfide (PPS) resin such that the
content of the PPS resin was 72 vol %.
Comparative Example 6
[0087] In Comparative Example 6, as shown in Table 5, as a soft
magnetic powder constituting the soft magnetic particles, an
iron-silicon alloy (Fe-6.5Si) powder containing 6.5 mass % of Si in
addition to Fe was prepared, the soft magnetic powder was kneaded
with an epoxy resin such that the content of the epoxy resin was 60
vol %, the kneaded material was put into a forming die having the
same size and the same shape as those of Example 1, and the epoxy
resin was cured. As a result, a ring test piece was prepared.
TABLE-US-00005 TABLE 5 Coating Conditions Base Content Density
Particles Resin (vol %) (g/cm.sup.3) Comparative Fe--6.5Si PPS 65
5.29 Example 4 Comparative Fe--6.5Si PPS 72 5.71 Example 5
Comparative Fe--6.5Si Epoxy 60 4.93 Example 6
<Measurement of Density of Ring Test Piece>
[0088] Regarding each of the ring test pieces of Examples 1 to 7
and Comparative Examples 1 to 6, the weight was measured, and the
density was measured from the volume during the formation. The
results are shown in the respective items of Tables 3 to 5. In the
ring test pieces of Comparative Examples 4 to 6, the content of the
resin was high, and thus the density was lower than that of
Examples 1 to 7 and Comparative Examples 1 to 3.
<Evaluation of Ring Test Piece>
[0089] Regarding each of the ring test pieces of Examples 2 to 7
and Comparative Examples 2 to 6, the magnetic flux density was
measured by applying a magnetic current until 60 kA/m using the
same method as that of Example 1. The first differential relative
permeability .mu.'L in an applied magnetic field of 100 A/m, the
second differential relative permeability .mu.'H in an applied
magnetic field of 40 kA/m, and .mu.'L/.mu.'H were calculated.
Further, .mu.'24 k/.mu.'L was also calculated by measuring a first
differential relative permeability .mu.'24 k in an applied magnetic
field of 24 kA/m. The results are shown in Table 6. The magnetic
flux density shown in Table 6 refers to the value in an applied
magnetic field of 60 kA/m.
[0090] Using the above results, FIG. 6 shows a relationship between
an applied magnetic field and a magnetic flux density in each of
the ring test pieces of Examples 1 to 7 and Comparative Examples 2
to 6. FIG. 7 shows a relationship between .mu.'L/.mu.'H and a
magnetic flux density B in an applied magnetic field of 60 kA/m in
each of the ring test pieces of Examples 1 to 7 and Comparative
Examples 1 to 6.
TABLE-US-00006 TABLE 6 Magnetic Flux Density B (T) .mu.'L .mu.'H
.mu.'L/.mu.'H .mu.'24k .mu.'24k/.mu.'L Example 1 1.33 45 11.1 4 18
0.40 Example 2 1.29 41 10.9 4 18 0.43 Example 3 1.63 77 10.2 8 18
0.24 Example 4 1.35 48 10.4 5 17 0.35 Example 5 1.50 60 10.6 6 18
0.30 Example 6 1.30 40 10.9 4 18 0.44 Example 7 1.30 40 10.8 4 18
0.45 Comparative 1.95 151 6.5 23 14 0.09 Example 1 Comparative 2.14
210 6.6 20 14 0.11 Example 2 Comparative 2.16 32 5.9 32 13 0.07
Example 3 Comparative 0.84 2 16 2 12 0.71 Example 4 Comparative
1.03 2 22 2 14 0.64 Example 5 Comparative 0.96 2 22 2 13 0.62
Example 6
[Result 3]
[0091] As shown in Table 6 and FIGS. 6 and 7, in the ring test
pieces of Comparative Examples 4 to 6, the content of the resin is
high, and thus the distance between the soft magnetic particles was
increased. As a result, it is considered that the resin was present
between the soft magnetic particles, and thus the magnetic flux
density in an applied magnetic field of 60 kA/m was lower than that
of Examples 1 to 7 and Comparative Examples 1 to 3.
[0092] In addition, regarding each of the ring test pieces of
Examples 1 to 7 and Comparative Examples 1 to 3, the magnetic flux
density in an applied magnetic field of 60 kA/m was secured to be
1.15 or higher. However, in the ring test pieces of Comparative
Examples 1 to 3, .mu.'L/.mu.'H exceeded 10 unlike in Examples 1 to
7. Therefore, as shown in Result 2, a decrease in differential
relative permeability in a high magnetic field is more concerned as
compared to Examples 1 to 7.
[0093] For a comparison to the technique disclosed in JP
2002-141213 A, Table 6 also shows the values of the first
differential relative permeability .mu.'24 k in an applied magnetic
field of 24 kA/m and .mu.'24 k/.mu.'L. The magnetic characteristics
of the dust core disclosed in JP 2002-141213 A are similar to those
of Comparative Examples 4 to 6 of the present application and are
clearly different from those of Examples 1 to 7. In addition, it is
considered that, as the .mu.'L/.mu.'H value of the dust core
disclosed in JP 2002-141213 A approaches the values of the ring
test pieces of Examples 1 to 7, the magnetic flux density in an
applied magnetic field of 60 kA/m is decreased.
<Measurement of Hardness and Thickness>
[0094] In the powder for a dust core used in each of the ring test
pieces of Examples 1 to 7 and Comparative Examples 1 to 3, the
hardness of the soft magnetic particles (base material) and the
hardness of the insulating film were measured. Specifically, these
materials were treated under the same conditions as shown in Tables
3 to 5 to prepare blocks, respectively. By measuring the hardness
of each block using a micro-Vickers hardness meter, the hardness of
the soft magnetic particle (base material) and the hardness of the
insulating film was obtained. Further, a ratio of the above
harnesses (the Vickers hardness of the insulating film/the Vickers
hardness of the base material) was calculated. The results are
shown in Table 7. Table 7 also shows the magnetic flux density B
(T) in an applied magnetic field of 60 kA/m and .mu.'L/.mu.'H shown
in Table 6.
[0095] In the powder for a dust core used in each of the ring test
pieces of Examples 2 to 7 and Comparative Example 1 to 3, the
thickness of the insulating film was measured using the same method
as that of Example 1. The results are shown in Table 7.
[0096] Using the above results, FIG. 8A shows a relationship
between .mu.'L/.mu.'H and the ratio of the hardness of the
insulating film of the powder for a dust core used in each of the
ring test pieces of Examples 1 to 7 and Comparative Examples 1 to
3. FIG. 8B shows a relationship between .mu.'L/.mu.'H and the
thickness of the insulating film of the powder for a dust core used
in each of the ring test pieces according to Examples 1 to 7 and
Comparative Examples 1 to 3.
TABLE-US-00007 TABLE 7 Magnetic Hardness Hard- Flux Hard- of Base
ness of Thick- Density .mu.'L/ ness Material Insulating ness B (T)
.mu.'H Ratio (Hv) Film (Hv) (.mu.m) Example 1 1.33 4 3.5 400 1400
500 Example 2 1.29 4 3.5 400 1400 500 Example 3 1.63 8 8 180 1400
400 Example 4 1.35 5 8 180 1400 400 Example 5 1.50 6 8 180 1400 400
Example 6 1.30 4 3.5 400 1400 400 Example 7 1.30 4 3.5 400 1400 200
Comparative 1.95 23 0.16 190 30 30 Example 1 Comparative 2.14 20
0.23 190 30 30 Example 2 Comparative 2.16 32 0.23 190 30 30 Example
3
[Result 4]
[0097] As shown in Table 7 and FIG. 8A, when the powders for a dust
core of Examples 1 to 7 were used, the .mu.'L/.mu.'H values of the
ring test pieces were 10 or lower. When the powders for a dust core
of Comparative Examples 1 to 3 were used, the .mu.'L/.mu.'H values
of the ring test pieces were 20 or higher.
[0098] The reason is considered to be as follows: since the
insulating film of each of the powders for a dust core of Examples
1 to 7 was significantly harder than the soft magnetic powder as
the base material, and thus the insulating film was held between
the soft magnetic particles during compression forming without
being moved. On the other hand, in each of Comparative Examples 1
to 3, the hardness of the insulating film was equivalent to that of
the soft magnetic powder as the base material. Therefore, as shown
in FIG. 2D, the insulating film was compressed at a triple point of
a boundary of the soft magnetic powder. Therefore, it is considered
that the .mu.'L/.mu.'H values of the ring test pieces were higher
than those of Examples 1 to 7. In Table 7, the hardness of the base
material and the hardness of the insulating film of Comparative
Example 1 were the same as those of Comparative Examples 2 and 3,
but the hardness ratios thereof were different from each other.
This result is caused by significant digits.
[0099] It is presumed from the above results that, when the
insulating film has a Vickers hardness which is 1.5 times or higher
than that of the soft magnetic particles as shown in FIG. 8B,
during compression forming, the movement of the insulating film to
the triple point between the soft magnetic particles can be
suppressed, and the relationship of .mu.'L/.mu.'H.ltoreq.10 can be
satisfied.
[0100] Further, in order to secure the above-described
characteristics, as shown in Table 7 and FIG. 8, it is preferable
that the thickness of the insulating film is 150 nm or more under
the condition of the above-described hardness ratio. It is
considered that, by securing the thickness of the insulating film,
the relationship of .mu.'L/.mu.'H.ltoreq.10 can be secured.
[0101] Hereinabove, the embodiment of the invention has been
described. However, a specific configuration is not limited to the
embodiment, and design changes and the like which are made within a
range not departing from the scope of the invention are included in
the invention.
* * * * *